The present invention is in the field of battery technology and, more particularly, in the area of solid electrolyte compositions for electrochemical cells.
Conventional lithium ion batteries include a positive electrode (or cathode as used herein), a negative electrode (or anode as used herein), an electrolyte, and, frequently, a separator. The electrolyte typically includes a liquid component that facilitates lithium ion transport and, in particular, enables ion penetration into the electrode materials.
In contrast, so-called solid-state lithium ion batteries do not include liquid in their principal battery components. Solid-state batteries can have certain advantages over liquid electrolyte batteries, such as improvements in safety because liquid electrolytes often contain volatile organic solvents. Solid-state batteries offer a wider range of packaging configurations because a liquid-tight seal is not necessary as it is with liquid electrolytes.
Generally, batteries having a solid-state electrolyte can have various advantages over batteries that contain liquid electrolyte. For small cells, such as those used in medical devices, the primary advantage is overall volumetric energy density. For example, small electrochemical cells often use specific packaging to contain the liquid electrolyte. For a typical packaging thickness of 0.5 mm, only about 60% of the volume can be used for the battery with the remainder being the volume of the packaging. As the cell dimensions get smaller, the problem becomes worse.
Elimination of the liquid electrolyte facilitates alternative, smaller packaging solutions for the battery. Thus, a substantial increase in the interior/exterior volume can be achieved, resulting in a larger total amount of stored energy in the same amount of space. Therefore, an all solid-state battery is desirable for medical applications requiring small batteries. The value is even greater for implantable, primary battery applications as the total energy stored often defines the device lifetime in the body.
Further, solid-state batteries can use lithium metal as the anode, thereby dramatically increasing the energy density of the battery as compared to the carbon-based anodes typically used in liquid electrolyte lithium ion batteries. With repeated cycling, lithium metal can form dendrites, which can penetrate a conventional porous separator and result in electrical shorting and runaway thermal reactions. This risk is mitigated through the use of a solid nonporous electrolyte for preventing penetration of lithium dendrites and enabling the safe use of lithium metal anodes, which directly translates to large gains in energy density, irrespective of cathode chemistry.
There has been considerable work done in the industry on solid-state electrolyte technologies and the state of the art materials typically fall into one of two categories: polymer solid-state electrolytes and inorganic solid-state electrolytes.
Regarding polymer solid-state electrolytes, they have certain advantages, such as being easily processable by standard solution casting techniques and having a flexible nature that allows the polymer to conform to electrode surfaces. Conformal coatings in turn can allow for good mechanical compliance and little loss of contact during battery cycling. On the other hand, polymer solid-state electrolytes have certain drawbacks, including relatively low conductivity (in a range of about 10−6 to about 10−5 S/cm) and relatively poor stability at when operated at high voltage (for example, polyethylene oxide polymers are commonly used and have poor high voltage stability). Also, relatively soft polymer films do not prevent the lithium dendrite penetration described above.
Regarding inorganic solid-state electrolytes, they have certain advantages such as relatively high conductivity (in a range of about 10−4 to about 10−3 S/cm for the state-of-the-art materials) and comparative hardness that can prevent lithium dendrite penetration. However, this hard and brittle nature of inorganic solid-state electrolyte materials makes them difficult to produce on an industrial scale, especially thin inorganic electrolyte films. The brittleness can lead to loss of contact with the electrode during battery cycling.
Within the class of inorganic solid-state electrolytes, a family of phosphates referred to as NASICON is an attractive candidate for use in batteries. NASICON is an acronym for sodium (Na) Super Ionic CONductor and usually refers to solid materials represented by the chemical formula Na1+xZr2SixP3−xO12, where 0<x<3. A state-of-the-art example of a lithium version of this material is any of several compositions similar to Li1.3Ti1.7Al0.3(PO4)3 (LTAP). This material has demonstrated good conductivity (on the order of about 10−4 S/cm). However, the titanium is electrochemically active at about 2.5V versus lithium and will spontaneously be chemically reduced when put in direct contact with a lithiated anode. The chemical reduction of the titanium in the LTAP can lead to lithium loss in the full cell and subsequent degradation of battery performance.
To account for the chemical reduction of titanium, titanium has been replaced with comparatively inactive zirconium (that is, zirconium is less susceptible to chemical reduction) in materials such as LiZr2(PO4)3 (LZP). However, LZP undergoes a low temperature phase transition from its conductive rhombohedral crystalline phase to a low conductivity triclinic crystalline phase at temperature in the range of from about 30 degrees Celsius to about 40 degrees Celsius (see, Arbi et al., Li mobility in triclinic and rhombohedral phases of the Nasicon-type compound LiZr2(PO4)3 as deduced from NMR spectroscopy, J. Mater. Chem., 2002, 12, 2985-2990). This temperature-induced crystalline phase transition significantly limits the practical use of LZP materials in a battery.
There has been some research into doping of LZP and other lithium materials, such as Barré, M., Le Berre, F., Crosnier-Lopez, M P. et al., The NASICON solid solution Li1-xLax/3Zr2(PO4)3: optimization of the sintering process and ionic conductivity measurements Ionics, (2009) 15: 681; Hui Xie, John B. Goodenough, Yutao Li, Li1.2Zr1.9Ca0.1(PO4)3, a room-temperature Li-ion solid electrolyte, Journal of Power Sources, Volume 196, Issue 18, 15 Sep. 2011, Pages 7760-7762; Yutao Li, Meijing Liu, Kai Liu, Chang-An Wang, High Li+ conduction in NASICON-type Li1+xYxZr2-x(PO4)3 at room temperature, Journal of Power Sources, Volume 240, 15 Oct. 2013, Pages 50-53; Mustaffa, N. A. & Mohamed, N. S., Zirconium-substituted LiSn2P3O12 solid electrolytes prepared via sol-gel method, J Sol-Gel Sci Technol (2016) 77: 585; and Russian Journal Of Inorganic Chemistry, Volume: 50 Issue: 6 Pages: 906-911. However, none of the prior art formulations address the significant limitations of current solid-state electrolytes and provides the performance improvements seen in the embodiments disclosed below.